The invention relates to gearboxes which equip particular systems, such as, for example, particular vehicles, possibly of the automotive type, and more precisely the synchronizers these gearboxes comprise.
As is known to those skilled in the art, a gearbox generally comprises at least one primary shaft, at least one secondary shaft, at least one synchronizer, at least one fork associated with one synchronizer, and at least one fork actuator. Each primary shaft and the associated secondary shaft are respectively provided with fixed pinions and idling pinions intended to define ratios (or speeds) together.
Each actuator is responsible for moving a fork so as to induce a displacement of a part of a synchronizer, partly translatable on a secondary shaft, in order to cause the secondary shaft to be coupled in rotation with the associated primary shaft, via an idling pinion of this secondary shaft.
For example, each synchronizer may comprise at least:                one hub secured in rotation to the secondary shaft by internal splines co-operating with external splines of this secondary shaft,        one sleeve (or sliding gear) mounted in translation on the hub,        one synchronizing ring comprising a female conical portion capable of being coupled to a male conical portion of a dog gear of an idling pinion, and        one arming mechanism installed between an inner face of the sleeve and an outer face of the hub, and for translating the synchronizing ring for coupling it to the dog gear of the idling pinion when the sleeve is translated by the associated fork.        
Each sleeve is capable of being translated, after angularly shifting the associated synchronizing ring during a phase generally known as “backspin,” so as to be coupled, during a phase generally known as “clutching”, to the idling pinion which has been previously synchronized by the synchronizing ring during a phase generally known as “synchronization”. The coupling of a synchronizing ring to an idling gear is intended to synchronize the speed of rotation of the idling gear with the speed of the secondary shaft which carries it, and the coupling of a sleeve to this same idling gear is intended to temporarily rotationally secure this idling gear to the secondary shaft which carries it.
Each actuator comprises a cam, the rotation of which causes the translation of a fork, and thus of the sleeve of a synchronizer. This cam has a height which must make it possible to systematically ensure the passage of a ratio (or speed) without jamming before the end of the stroke. This height depends directly on the stroke of the sleeve and must be adjusted as best possible to ensure a margin of minimum distance (or translation) mdm in grip.
As shown in FIG. 1, the aforesaid (mdm) is predefined in such a way that a narrow coupling of the inner splines CI2 of the sleeve MA at the dogs CR of the dog gears CC of the idling pinion PF1 can occur, taking into account the dimensional variations in the manufacture of the sleeve MA, the synchromesh ring and the idling gear PF1. More precisely, the inner splines CI2 of the sleeve MA comprise end teeth, comprised of so-called “anti-release” indents RE, in which there are to be housed protuberances PR, defined at the intersection between dihedrons constituting the dogs CR of the dog gear, in order to ensure the effective engagement of the idling pinion PF1. The margin of minimal distance (or translation) MDM in grip is then defined as the minimum distance between the front end of an anti-release indents RE and the intersection of the dihedrons, when the ratio is engaged and the sleeve MA has covered the dog gear CC.
The cam height depends on the gearbox design and the synchronizers to be driven (number of cones, angularity of the teeth ends or dogs). Consequently, for the same actuator ramp to control all the synchronizers, the determination of the dimension chains proves to be complex in order to avoid drag and “heat” problems of the synchronizers. It is to be recalled that the phenomena of drag of the gearbox is controlled when the dimension chains of the idling gear play is typically between approximately 0.06 mm and 0.3 mm.
It is therefore difficult to manage the height of this cam for gearboxes which are mass produced, for example for passenger cars, mainly because of tight manufacturing dimensional tolerances and harmonization of the chains of dimensions of the synchronizers to be driven. Indeed, if it is possible to accomplish precise pairing of the forks by machining or by fitting with shims for a competition gearbox, it becomes impossible to achieve for a standard gearbox, due to the costs, the control time and the accuracy of the metrology.
It will be noted that this operation is all the more difficult to achieve when the dimensions of the gearboxes are reduced in order to make them more compact.